During drilling of a wellbore through subterranean rock formations, fluids are used for various purposes. The fluids cool and lubricate the drill string, carry drill cuttings to the surface, provide sufficient hydrostatic pressure to inhibit the ingress of formation fluids into the wellbore, etc. Drilling fluids may also contain additives, for example, fluid loss control agents to prevent the loss of drilling fluid into pores or fractures preexisting in the rock formation or induced by drilling action.
Apparatus for testing the efficacy of drilling fluid additives as fluid loss control agents are known. Such apparatus typically comprise a filter press including a pressure cell with a fluid inlet, a fluid outlet and a fluid-permeable medium secured within the pressure to simulate rock formation pores or fractures and so predict whether certain additives are capable of entering fractures in susceptible formations to plug the fracture. The fluid-permeable medium may be a ceramic filter disk that is secured in the outlet cap of the pressure cell. Typically, a plurality of filter disks is provided that vary in porosity so that differently-sized pores or fractures can be simulated. Such a testing device is available from, e.g., OFI Testing Equipment, Inc., of Houston, Tex., as the “Permeability Plugging Tester,” (for which the Instruction Manual is attached to this disclosure as Appendix A). See also, e.g., US 2011/0295509, and WO 2008/112795, both of which are incorporated by reference herein. Machined disks with opening slots, such as those illustrated in SPE 124087, are often used to simulate fractures that are 200 microns or larger.
By way of the present disclosure, an assembly is provided for simulating fractures in a wellbore to be used in combination with the pressure cell of a filter press in place of the ceramic filter disks and machined disks described above. More particularly, a fracture simulation assembly is provided in which the constituent parts may be selected to simulate fractures of varying size.